Immobilized catalytically-active-molecules such as enzymes have been used as binding participants to determine the presence or amount of the immobilized enzyme's substrate that may be present in a test sample. Commercial chemistry analyzers use enzymes, which have been bound to porous surfaces, for the conversion of enzyme substrates to optically or electrochemically detectable products. For example, electrochemical sensors utilize the ability of an immobilized enzyme to form an electrochemically active molecule as a result of the action of the enzyme on its substrate. Such sensors employ a potentiostat and amperometric electrodes that typically consist of an enzyme or working electrode, a reference electrode and a counter electrode. The enzyme electrode is typically made of platinum and is supplied with an overlaying oxidase enzyme layer. When the surface of an electrode is immersed in a sample containing an oxidizable substrate and molecular oxygen, both molecules diffuse into the enzyme layer where the substrate reacts with the enzyme resulting in reduction of the enzyme. The reduced enzyme is oxidized by the molecular oxygen which, in turn, is reduced to peroxide. At a sufficiently high electrode potential (maintained via the reference electrode), the platinum portion of the enzyme electrode oxidizes the peroxide to regenerate oxygen and transfer two electrons to the counter electrode. The potentiostat measures the current generated by the transferred electrons and the amount of current is related to the amount of oxidizable substrate in the sample. Hence, the presence and/or amount of an oxidizable substrate in the sample can be determined.
Prior to the present invention, devices which electrochemically or optically detect the presence or amount of an analyte which may be present in a test sample are generally single use disposable devices which are incapable of analyzing a plurality of circulating test samples.